L,N = map(int,input().split())
X = list(map(int,input().split()))
D = [0]*(N+1)
D[0] = X[0]
D[N] = L-X[N-1]
for i in range(1,N):
    D[i] = X[i]-X[i-1]
dp = [[False for _ in range(L+1)] for _ in range(N+1)]
for i in range(N):
    dp[i][0] = True
for i in range(1,N):
    for j in range(1,L+1):
        dp[i][j] = dp[i-1][j]
        if j>=D[i]:
            dp[i][j] = dp[i][j] or dp[i-1][j-D[i]]
ans = 10**5
for j in range(L+1):
    if dp[N-1][j]:
        ans = min(ans,abs(2*D[0]+2*j-L))
print(ans)